The field of the invention relates to lean burn engine control in internal combustion engines.
lean burn engine systems can have different cylinder groups, each having a close-coupled catalytic converter. These cylinder groups come together in a y-pipe configuration before entering a under-body catalyst. The catalyst can store oxidants (including NOx) when operating lean, and release and reduce the oxidants with incoming reductants when operating rich. In this way, emissions are minimized while operating lean by also periodically operating rich. One such system is described in U.S. Pat. No. 5,970,707. In this system, lean and rich operation of the cylinder groups is generally synchronized during normal operation.
The inventors herein have recognized that while the Y-type configuration has some advantages, there may not be enough freedom to optimize exhaust system tuning. In particular, the underbody catalyst typically places a constraint on the location of the Y-pipe to provide optimal temperature window operation for the underbody catalyst.
On the other hand, the inventors herein have also recognized that having a dual exhaust system where two underbody catalysts are used with a Y-pipe joining them afterwards, provides more flexibility in positioning the Y-pipe joint. Therefore, there is more freedom for optimizing the exhaust system tuning.
Finally, the inventors herein have recognized that maintaining synchronous lean and rich engine operation of the dual catalyst path system may not fully use the catalyst""s storage ability. In particular, due to component variation of the underbody catalysts, bank to bank variation of engine exhaust gas properties, and different aging rates of components, the catalysts on the different banks may not behave identically. The potential difference in catalyst conversion and storage/regeneration, if coupled with synchronous operation of the banks between lean and rich air fuel ratios, may therefore lead to degraded performance. For example, one catalyst may finish releasing or reducing stored NOx and oxygen before the other one does. In this case, if the rich operation of the two banks continue, there may be hydrocarbon and carbon monoxide break through from the catalyst that has already completely released stored oxidants. If the rich operation stops, on the other hand, the storage capacity of the other catalyst may not be fully regenerated, thereby leading to degraded performance in subsequent operation. In either case, the fuel economy and emissions may be negatively impacted.
Disadvantages of prior approaches are overcome by a method for controlling an engine having a first and second group of cylinders, the first group coupled to a first catalyst and the second group coupled to a second catalyst. The method comprises: concurrently operating the first and second cylinder groups rich of stoichiometry; in response to a first indication that said rich operation of at least one of the first and second catalysts should be ended, operating the group coupled to the at least one catalyst near stoichiometry while continuing operation of the other group rich of stoichiometry; and in response to a second indication that said rich operation of the other catalyst should be ended, ending rich operation of the other group. By operating the cylinder group coupled to the catalyst that has depleted stored oxidants near stoichiometry, HC and CO breakthrough are minimized while at the same time minimizing any torque imbalance between the two cylinder groups, i.e., since one bank is operating rich and the other near stoichiometry (with the same amount of air per cylinder), engine torque is substantially maintained since the additional fuel in the rich cylinder does not burn to make torque. Any slight torque increase in torque can be compensated for by ignition retard on the rich cylinder bank. In this way, the other catalyst can also be depleted of stored oxidants. Therefore, the full potential of both catalysts is achieved without sacrificing emission performance or driveability.
An advantage of the above aspect of the invention is therefore improved emissions and more efficient use of catalysts in separate exhaust streams.
Also note that the indications provided above may be given in a variety of ways such as based on air-fuel ratio sensors coupled downstream of the catalyst, based on estimates using other operating parameters, or various other indications.
Other advantages of the present invention will be readily appreciated by the reader of this specification.